Study On Synthesis Of Multifunctional Graphene/polyimide Nanocomposite Sponges And Their Applications

Graphene nanosheet exhibits exceptional and fascinating mechanical,electrical,optic and thermal properties,which endow it great potential as a unique nanoscale building block for the construction of macroscopic monoliths.Moreover,threedimensional?3D?graphene sponges exhibit super-low density,multiscale porous structure,and excellent electrical properties,which attact considerable attention.However,these reported graphene foams of aerogels undergo significant plastic deformation or brittle mechanical performance when suffering from strain.This results restrict the widespread applications of 3D graphene materials.Constructing multifunctional graphene foam with excellent mechanical properties is s till a big challenge.In this paper,water-soluble polyimide?PI?was introduced to enhance the elastic and flexible properties of graphene foams.Graphene-polyimide nanocomposite foams,highly ordered graphene-polyimide thermally insulating foams and anisotropic graphene-polyimide thermally insulating foams with hollow spherical structures were preparated and characterized to explore their potential application prospects in strain sensors or thermally insulating fields.With the introduction of water-soluble PI,light-weight and mechanically flexible graphene-polyimide nanocomposite were synthesized by freeze-drying and subsequent thermal annealing.The synergistic effect between graphene nanosheets and PI building blocks endows the nanocomposite with good electrical conductivity and super-elastic.The nanocomposite can give a quick response of resistance variation ratio under the deformation of compression,bending,stretching,and torsion.Graphene-polyimide nanocomposite can undergo 2000 times cyclic compression with only 3% residual strain.Also,the composites express the Young modulus of 1.738 MPa with high tensile strain?15.6%?.Graphene-polyimide nanocomposite exhitbits high pressure sensitivity and electromechanical stability,holding great potential for the further applications as light-weight,super-elastic multifunctional strain sensors.Highly ordered graphene-polyimide thermally insulating foams were fabricated by freeze-casting.Single layer graphene nanosheets exhibit high thermal conductivity.However,the 3D graphene foam can be used as an insulating material on account of phonon scattering effects.With the increase of graphene content,the anisotropic microstructure of nanocomposite is formed.The strong orientation effect of the controlled structures results in significant anisotropic mechanical properties?16.2 k Pa and 8.6 k Pa in axial and radial direction at ?=50 %,respectively?.Moreove,the increasing graphene content results anisotropic thermal properties.The thermal conductivity in axial direction increases gradually(from 0.026 W?m-1?k^-1 to 0.038 W?m-1?k^-1),while the thermal conductivity in radial direction decreases(from 0.021 W?m-1?k^-1 to 0.012 W?m-1?k^-1).The insulating mechanisim of nanocomposite was also explored by the investigation between the anisotropic structure and thermal conductivity.Based on the anisotropic structures of the thermally insulating foams,the hollow spherical structures were introduced to improve the insulating property.Modified polystyrene?PS?spheres worked as sacrificial templates to form the graphene coated hollow structures by thermal annealing.The addition of hollow spherical sturctures exceptionally strong impede the thermal transport in the radial orientation of the thermally insulating nanocomposite foam(as low as 0.009 W?m-1?k^-1).Also,the nanocomposite foams are applicable and effective as a thermal insulator in cold environments.